The world’s largest living thing is a stealthy parasite that lives mostly underground and beneath the bark of infected trees in a pale, stringy fungal network. COURTESY MIKE MCWILLIAMS

Thanks to Milo‘s interest in fungi, which we found infectious, we learned years ago what the world’s largest living thing is. We used to feature more stories from Atlas Obscura, but this is the first in a few years:

What the World’s Largest Organism Reveals About Fires and Forest Health

The Humongous Fungus is most visible when it produces the edible and tasty honey mushroom, but the season is brief and doesn’t happen every year. PETER PEARSALL, USFWS/PUBLIC DOMAIN

In Oregon, the Humongous Fungus plays a complex role in an ecosystem reshaped by humans.

UNDER THE BLUE MOUNTAINS OF Oregon lurks something massive and prehistoric. Yet the largest recorded organism on Earth, weighing more than 200 blue whales and dwarfing even Pando, Utah’s famous grove of quaking aspens, is nearly invisible to the untrained eye. It’s a single, genetically identifiable specimen of honey mushroom, or Armillaria ostoyae, that has been growing for thousands of years.

Nicknamed the Humongous Fungus, it covers nearly four square miles within Malheur National Forest and weighs perhaps 7,500 tons (some estimates range as high as 35,000 tons). The fungus likely attained its record-setting dimensions in part thanks to conditions created by 20th century forest management. And it continues to grow, expanding mostly underground in networks of thin filaments called mycelia. As the fungus spreads, it moves up into trees, hidden beneath their bark. It then slowly eats away at its host, often killing the tree and then continuing to munch on the dead wood for decades. More than just an insidious parasite, the Humongous Fungus is a symbol of an ailing, at-risk forest, unintended consequences of fire suppression, and the challenge of restoring an ecosystem’s health.

“If there were no trees dying, I wouldn’t have a job,” says forest pathologist Mike McWilliams, who calls himself the unofficial tour guide of the massive fungus. “But I like this thing because it’s super interesting.”

McWilliams, whose official duties center around conservation efforts at Malheur, meets visiting researchers (and the occasional curiosity seeker) along U.S. Highway 26, where a country store under towering pines advertises its famous huckleberry ice cream and buffalo burgers. From there, he leads the way along one Forest Service gravel road and then another. Eventually, the party must get out to hike.

Soon, dense forest gives way to a balding hillside. The few trees here are more spread out, and some are clearly dying—the work not of the Humongous Fungus, but a smaller relative. In Malheur, there are several different armillaria specimens, and it’s hard to tell with boots on the ground where one fungus ends and another begins. So researchers collect samples and map them genetically…

Read the whole story here.